Magnetographic printing machines are known which in response to the reception of electrical signals from a control unit make it possible to form images, for instance images of characters, on a printing substrate, typically a sheet or strip of paper. In these printers, which are similar to those described and shown in U.S. Pat. Nos. 3,161,544 and 4,072,957, printing of the images is attained by first forming a latent magnetic image, based on the signals received, on the surface of a magnetic recording element. The recording element is coated with a film of magnetic material and is generally in the form of a rotating drum or an endless belt. The latent magnetic image is then developed, or in other words made visible, with the aid of a powdered developer comprising particles of thermoplastic resin enclosing magnetic particles and pigments, which is attracted only by the regions of the recording element on which the latent image has been recorded. The developer then forms an image in powder on the surface of the element, and this powder image is then transferred to the printing substrate.
In order to permit the formation of the latent magnetic image on the surface of the recording substrate, these machines are provided with a recording device known as a transducer, which includes one or more magnetic recording heads into proximity with which the recording element is displaced. Each of these heads generates a magnetic field, whenever it is excited for a brief moment by an electric current of suitable intensity, creating magnetized domains of small dimensions on the surface of the recording element. These domains are virtually punctiform and are generally known as magnetized points.
A set of these magnetized points comprises the latent magnetic image. The portion of the surface of the recording element that passes before each head is conventionally known as the data recording track. The recording element generally includes a plurality of tracks, which may be subjected to recording either individually, in the course of successive recording operations, or simultaneously in the course of a single operation.
Magnetographic printing machines have already been made in which the transducer includes as many magnetic heads as there are tracks on the recording element. The heads are disposed side by side and are aligned along a direction transverse to the direction of displacement of the recording element. Since in these machines each track is associated with each of the transducer heads, respectively, recording of a latent image on the recording element is accomplished in the course of a single displacement revolution of this element along its endless orbit. Accordingly, these machines are capable of functioning at a high printing speed, which may for example be as high as a hundred pages per minute. Nevertheless, for certain applications, such high speed is not always necessary, so that a less-powerful magnetographic printer, that is also less expensive, equipped with a transducer that includes a number of magnetic heads notably less than the number of tracks of the recording element, may be sufficient. Such a magnetographic printing machine is known from U.S. Pat. No. 4,072,957, where the transducer includes only a single magnetic head, which is mounted in such a way that it can be displaced along a magnetic recording drum in the direction parallel to the axis of rotation of the drum.
In this known machine, recording of a latent magnetic image is performed track by track. Recording of the data in a track located facing the head is performed in the course of one complete revolution of the drum. At the end of this revolution, the head has been displaced so that it is facing the following track and allows recording of this following track in its turn. Under these conditions, the recording of the latent image on the drum is performed in as many revolutions of the drum as there are tracks on the drum. The development of the latent image, that is, the depositing of particles of developer onto the drum, is not undertaken until the formation of the image on the drum is completed. This operation is performed by means of an applicator device of a known type, which in the machine described in the aforementioned U.S. Pat. No. 4,072,957 includes a magnetic cylinder mounted on a shaft parallel to the axis of rotation of the drum. This cylinder, placed in proximity with the surface of the drum, is disposed in such a way as to be in contact with the developer particles contained in a reservoir placed beneath the drum. Thus when the magnetic cylinder revolves, the developer particles, which are driven to rotate by this cylinder, are moved to the vicinity of the surface of the drum and upon being attracted by the magnetized points on the surface are deposited on the portions of the surface on which the latent image has been formed. The particles thus deposited then travel past a transfer roller, which is normally pressed against the surface of the drum, and are thus transferred to a sheet of paper that at that moment is engaged between the drum and the transfer roller.
In this applicator device, the magnetic cylinder is not driven to rotate continuously but rather only for one revolution of the drum, following the formation of a latent image on the drum. Thus, during the periods of formation of latent images when no sheet of paper is engaged between the drum and the transfer roller, developer particles are prevented from being deposited on the drum and so do not soil the transfer roller. This applicator device, which functions intermittently and makes it possible to apply developer particles to the drum without causing clouds of particles capable of causing pollution inside the machine; however, it is still not completely satisfactory, because the drum is located a very slight distance away from the magnetic cylinder, and when the particles travel past the cylinder, the magnetized points that have been formed on the drum are necessarily exposed to the action of the magnetic flux generated by the cylinder, with the risk that they will be greatly altered or even erased.
Certainly, this disadvantage could be overcome by using an applicator device described in French Patent No. 2.408.462, which includes both a reservoir disposed below the recording element and containing developer particles and also a transport element arranged to place these particles in the vicinity of the surface of the recording element. The applicator device also includes a fixed deflector, interposed between the surface and the transport element to gather the particles transported by the transport element and arranged so that with this surface it forms a substantially prismatic spout, in which the gathered particles accumulate. The accumulated particles finally come into contact with the surface and are entrained by it in the direction of the apex of the prism comprising the spout, and the particles driven beyond this apex remain applied only to the magnetized points formed on the surface.
This applicator device, which causes no alteration whatever of the magnetized points and generates no pollution whatever inside the machine, nevertheless has the disadvantage of not assuring good development of the latent images when the transport element with which it is provided is driven intermittently rather than continuously.